US8768169B2 - Time synchronization method and system for a passive optical network system - Google Patents

Time synchronization method and system for a passive optical network system Download PDF

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US8768169B2
US8768169B2 US13/141,330 US200913141330A US8768169B2 US 8768169 B2 US8768169 B2 US 8768169B2 US 200913141330 A US200913141330 A US 200913141330A US 8768169 B2 US8768169 B2 US 8768169B2
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time
olt
onu
information
synchronization
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Liquan Yuan
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ZTE Corp
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
    • H04J3/0658—Clock or time synchronisation among packet nodes
    • H04J3/0661—Clock or time synchronisation among packet nodes using timestamps
    • H04J3/0667—Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/12—Arrangements providing for calling or supervisory signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0682—Clock or time synchronisation in a network by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging

Definitions

  • the invention relates to the technical field of Passive Optical Networks (PONs), in particular to a time synchronization method and a synchronization system for a PON system.
  • PONs Passive Optical Networks
  • the time synchronization in communication systems is to coordinate the time of communication equipment with the universal time, also called phase synchronization.
  • a communication network especially wireless devices, requires the synchronization among communication systems.
  • Table 1 lists the specific requirements of wireless devices on the synchronization:
  • the Passive Optical Network (PON) technology is a point to multipoint optical access technology, and a PON consists of an Optical Line Terminal (OLT) on the office side, an Optical Network Unit (ONU) on the user side and an Optical Distribution Network (ODN).
  • the PON has multiple types, and can be roughly divided into a wavelength division PON, a power PON and a hybrid PON combined by the two types above.
  • the power PON is further divided into an ATM (asynchronous transfer mode) Passive Optical Network (APON), a Gigabit Passive Optical Network (GPON) and an Ethernet Passive Optical Network (EPON) based on different protocols of link layers.
  • ATM asynchronous transfer mode
  • APON asynchronous transfer mode
  • GPON Gigabit Passive Optical Network
  • EPON Ethernet Passive Optical Network
  • the power PON generally adopts a Time Division Multiplex (TDM) broadcast mode and a Time Division Multiple Access (TDMA) mode for the downlink and uplink thereof respectively, and can flexibly constitute tree, star and bus and other topological structures (typically, the tree structure).
  • TDM Time Division Multiplex
  • TDMA Time Division Multiple Access
  • hybrid PON refers to that multiple power PONs are available in an ODN, and adopt different wavelengths.
  • FIG. 1 illustrates a diagram of networking and clock transfer of a PON in the field of mobile communications, and typical networking of the PON in the mobile communications is shown as FIG. 1 . It is needed for the PON to complete the transfer of two tasks: service stream transfer and clock synchronization transfer, and FIG. 1 shows the transfer and distribution mode of a clock stream.
  • the PON protocol only realizes the frequency synchronization per se, i.e. the synchronization of an ONU to an OLT; however, the phase difference between ONUs is random, i.e. incapable of supporting the time synchronization, thereby not realizing the mobile communications or the network utilization that requires the time synchronization.
  • IEEE1588 can realize the time synchronization among network devices that
  • a master and slave mode is used to tag a timestamp on a packet, and periodically distribute a clock, and a receiver measures the clock offset and the delay;
  • a point-to-point link can provide the highest accuracy, and a boundary clock is introduced, independent of delay jitter, and can reach the time accuracy of 10 ns every hop;
  • FIG. 2 illustrates a diagram of the implementation mechanism of the IEEE1588 protocol.
  • the Slave device sends a Delay_Req (delay request) message to the Master device at the local time of T3;
  • the IEEE1588 protocol is based on an assumption of the network characteristic symmetry, i.e. the delay and jitter from the Master device to the Slave device is the same with those from the Slave device to the Master device, and the accuracy of time synchronization is directly depended on the jitter magnitude of a link.
  • the GPON (i.e. a PON defined by the ITU G,984.1-4 specifications) bearing an Ethernet service refers to that the GPON bears a protocol stack of an Ethernet protocol packet, as shown in FIG. 3 .
  • a GPON encapsulation method (GEM) layer can segment and recombine packets for an upper layer service data stream, so that a message of the upper layer data stream is possibly segmented into multiple parts, each of which is sent at unfixed and different time, and a receiving terminal detects that whether all the segmented parts of a data packet have arrived, and, if so, implements an encapsulation to recover a complete upper layer data message.
  • GEM GPON encapsulation method
  • the GPON is a delay-varying network, i.e. the jitter thereof is large, so that the IEEE1588 protocol cannot run on the GPON directly. Therefore, the accurate time synchronization of the ONU with the OLT cannot be guaranteed in the PON.
  • the objective of the invention is to provide a time synchronization method for a PON system to solve the problem that an ONU cannot be synchronized with an OLT accurately in time in the PON.
  • a time synchronization method of a PON for implementing time synchronization from an Optical Line Terminal (OLT) to an Optical Network Unit (ONU) comprises:
  • time information includes a transmission delay between the OLT and the ONU and the sending time identifier information of the time information
  • the management path may be a Physical Layer Operation Administration Maintenance (PLOAM) message path; and the sending time identifier information of the time information may be the sending time of a PLOAM message.
  • PLOAM Physical Layer Operation Administration Maintenance
  • the management path may be an ONU Management and Control Interface (OMCI) message path; and the sending time identifier information of the sending time may comprise the sending time of a frame triggering the time synchronization and the frame number of the frame.
  • OMCI ONU Management and Control Interface
  • the time offset between the ONU and the OLT may be the difference between the local time of the ONU and the sum of the sending time of the time information and the transmission delay.
  • the information transmission delay may be obtained by the distance measuring function of the PON system, or by the measurement through simulating the IEEE1588 protocol by using the PLOAM message path.
  • the invention further provides a time synchronization system for the PON, comprising:
  • an OLT time synchronization receiving and processing module used for realizing time synchronization with a superior device
  • an OLT time information sending module used for calculating a transmission delay between an OLT and an ONU and transferring time information to the ONU by using a management path, wherein the time information comprises the transmission delay between the ONU and the OLT and the sending time identifier information of the time information
  • an ONU time synchronization receiving and processing module used for receiving time information from the OLT, calculating a time offset between the ONU and the OLT according to the time information and implementing the time synchronization with the OLT.
  • the OLT time information sending module may transfer the time information to the ONU by using a PLOAM message path, and the sending time identifier information of the time information may be the sending time of a PLOAM message.
  • the OLT time information sending module may transfer the time information to the ONU by using an OMCI message path, and the sending time identifier information of the time information may comprise the sending time of a frame triggering the time synchronization and the frame number of the frame.
  • the time offset may be the difference between the local time of the ONU and the sum of the sending time of the time information and the transmission delay.
  • the information transmission delay may be obtained by the distance measuring function of the PON system, or by the measurement through simulating the IEEE1588 protocol by using the PLOAM message path.
  • the invention realizes the transfer of time in the PON section by the specific PLOAM message path or OMCI message path of a GPON, and a PLOAM message or an OMCI message is not segmented during the transmission thereof in the PON section, thus avoiding the technical defect of the unfixed delay for an upper layer protocol data message in the GPON and guaranteeing that every ONU can be synchronized with the OLT accurately.
  • FIG. 1 is a schematic diagram illustrating the networking and clock transfer of a PON in the field of mobile communications
  • FIG. 2 is a schematic diagram illustrating an implementation mechanism of the IEEE1588 protocol
  • FIG. 3 illustrates a protocol stack of an Ethernet protocol packet bore by a GPON
  • FIG. 4 is a flowchart illustrating a time synchronization method for a PON according to an embodiment of the invention
  • FIG. 5 is a block diagram illustrating a time synchronization system for a PON according to an embodiment of the invention.
  • FIG. 6 is a flowchart illustrating a time synchronization method for a PON according to a preferred embodiment of the invention.
  • FIG. 4 is a flowchart illustrating a time synchronization method for a PON according to an embodiment of the invention, in which, a time synchronization method for a PON is provided, including: implementing time synchronization from an OLT to an ONU, further including the following steps:
  • the time information includes a transmission delay between the OLT and the ONU, and the transmission delay can be obtained by the distance measuring function of a PON system, or by the measurement through simulating the IEEE1588 protocol by using a Physical Layer Operation Administration Maintenance (PLOAM) message including the information identical to the IEEE1588 protocol; and
  • PLOAM Physical Layer Operation Administration Maintenance
  • the management path can be a PLOAM message path or an ONU Management and Control Interface (OMCI) message path;
  • OMCI ONU Management and Control Interface
  • the time information further includes the sending time of a PLOAM message, i.e. the local time of the OLT when the PLOAM message is sent; the ONU calculates to obtain the time offset between the OLT and the ONU based on the sending time of the PLOAM message which is included in the received PLOAM message and, the transmission delay or the information for calculating to obtain the transmission delay, and implements the frequency synchronization and the time synchronization based on the time offset obtained by calculation.
  • a PLOAM message i.e. the local time of the OLT when the PLOAM message is sent
  • the ONU calculates to obtain the time offset between the OLT and the ONU based on the sending time of the PLOAM message which is included in the received PLOAM message and, the transmission delay or the information for calculating to obtain the transmission delay, and implements the frequency synchronization and the time synchronization based on the time offset obtained by calculation.
  • the PLOAM message includes the information of second and nanosecond.
  • the PLOAM message containing time information is required to be sent periodically.
  • the time information further includes the sending time for sending a frame triggering time synchronization by the OLT and the frame number of the frame.
  • the ONU calculates a time offset (“Offset”) between the OLT and the ONU according to the sending time of the frame and a transmission delay, and implements frequency synchronization and the time synchronization based on the time offset obtained by calculation.
  • Offset a time offset
  • the synchronization method according to the embodiment of the invention may further includes: implementing time synchronization between the OLT and a superior device of the OLT by using the method of the IEEE1588 protocol before the OLT sends a message in the form of PLOAM to the ONU.
  • time synchronization is implemented between the ONU and an inferior device of the ONU by using the method of the IEEE1588 protocol. Therefore, the time synchronization between the inferior device connected with the ONU and superior network connected with the OLT is guaranteed, and the relay transmission of time is realized in the PON section.
  • an IEEE1588 clock protocol runs between the OLT and the superior device, and between the ONU and the inferior device, and the transfer of time is realized by periodically using a message in the form of PLOAM between the OLT and the ONU in the PON section of the OLT and the ONU.
  • the embodiment is applicable to the PON section, and implements time synchronizations between the superior device and the OLT, and between the inferior device and the ONU.
  • the embodiment of the invention realizes the transfer of time in the PON section by using the specific PLOAM message path or the OMCI message path of the GPON.
  • the PLOAM message or the OMCI message is not segmented during the transmission thereof.
  • the PLOAM message is sent as a whole to avoid the GPON characteristic of the unfixed delay for an upper layer protocol data message.
  • the synchronous transfer of time is realized in the PON and guarantees that every ONU has the same time. If a standard IEEE1588 protocol runs between the ONU and an inferior device of the ONU, the ONU can also transfer the time to the inferior device accurately and guarantee the same time among the inferior devices of every ONU.
  • FIG. 5 is a block diagram illustrating a time synchronization system for a PON according to an embodiment of the invention.
  • the system includes: an OLT time synchronization receiving and processing module, an OLT time information sending module, an optical fibre system for transferring time information and an ONU time synchronization receiving and processing module, wherein:
  • the OLT time synchronization receiving and processing module is used for realizing the time synchronization with a superior device, regulating the local time according to the time information sent by the superior device, thereby synchronizing the time of the OLT with that of the superior device accurately.
  • the OLT time information sending module is used for calculating a transmission delay between the OLT and the ONU and transferring the time information through a management path.
  • the OLT may also send information for calculating the transmission delay between the OLT and the ONU to the ONU which calculates the transmission delay between the OLT and the ONU.
  • the management path can further bear the time information by using a PLOAM message in the PON system.
  • the time information includes the sending time of the PLOAM message and the transmission delay between the OLT and the ONU or the information for calculating and obtaining the transmission delay.
  • the management path can further use an OMCI path of the GPON system.
  • the time information includes the transmission delay between the OLT and the ONU, the sending time of sending a frame triggering time synchronization by the OLT and the frame number of the frame.
  • optical fibre system for transferring time information is a point to multipoint optical fibre system serving as a physical medium for bearing the time information.
  • the ONU time synchronization receiving and processing module is used for receiving time information from the OLT and calculating a time offset between the OLT and the ONU according to the time information, thereby synchronizing the local ONU time.
  • the embodiment adopts the method of periodically sending a time synchronization message containing the time information between an OLT and an ONU, thus solving the problem that the ONU synchronized with the OLT accurately in time cannot be guaranteed in the PON, furthermore, achieving the time synchronization in the PON.
  • FIG. 6 is a flowchart illustrating a time synchronization method for a PON according to a preferred embodiment of the invention.
  • the method for realizing the time synchronization mechanism in the PON of the preferred embodiment specifically includes that:
  • a standard IEEE1588 protocol runs between an OLT and a device connected to the OLT; a superior device runs in the Master mode; the OLT runs in the Slave mode and is synchronized with the superior device accurately in time.
  • a time offset between the “superior device” and the OLT device is “Offset”
  • the “superior device” sends a Synchronization (Sync) message with a timestamp to the OLT at the time of T1, and the OLT locally receives the synchronization message at the time of T2.
  • Sync Synchronization
  • T 2 T 1+Delay+Offset (1)
  • the “superior device” sends a Follow_up message to the OLT;
  • the OLT sends a Delay_Req message to the “superior device” at the local time of T3;
  • T 4 T 3+Delay ⁇ Offset (2)
  • the Sync, follow_up, Delay_Req and Delay_Resp messages are periodically sent between the “superior device” and the OLT to dynamically update the value of the “Offset”.
  • the dynamic and real-time compensation for the “Offset” in the OLT can synchronize the OLT with the “superior device” in time.
  • master and backup clock sources can be realized, the network reliability can be provided, and the selection of the master and backup clock sources can be realized according to the IEEE1588 protocol.
  • the standard IEEE1588 protocol does not run between the OLT and the ONU.
  • the OLT device runs in the Master mode of the clock, and the ONU runs in the Slave mode of the clock and is synchronized with the OLT accurately in time.
  • the ONU triggers the local time synchronization with OLT at the time of T2, while the time offset (“Offset”) between the OLT and the ONU is the difference between the local clock of the ONU and T2, and the ONU regulates the local clock according to the value of the “Offset”.
  • the “Delay” can be obtained by the inherent distance measuring method of the GPON.
  • the “Delay” can also be obtained by simulating the 1588 protocol, i.e. simulating the delay measuring process of the 1588 by using a PLOAM message, and the specific method is the same with the method for measuring a point-to-point link by the 1588.
  • Every ONU is synchronized with the OLT, so that accurate time synchronization among every ONU is realized.
  • the standard IEEE1588 protocol runs between the ONU and an “inferior device” which is capable of realizing accurate time synchronization with the ONU, and the “inferior device” is a base station in the field of mobile communications.
  • the protocol messages transferring the time are transmitted and received by hardware to guarantee the time accuracy.
  • modules or steps of the present invention can be implemented by general computing devices and centralized in a signal computing device or distributed in a network consisting of multiple computing devices.
  • the modules or steps can be implemented by program codes executable by the computing devices, so that they can be stored in a storage device and executed by the computing devices, or respectively made into integrated circuit modules, or some of them can be made into a single integrated circuit module.
  • the present invention is not limited to any specific combination of hardware and software.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Small-Scale Networks (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
  • Time-Division Multiplex Systems (AREA)
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CN200910105318A CN101795423A (zh) 2009-02-04 2009-02-04 无源光网络系统的时间同步方法及其同步系统
PCT/CN2009/075787 WO2010088830A1 (zh) 2009-02-04 2009-12-21 无源光网络系统的时间同步方法及其同步系统

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EP2372932A4 (en) 2014-05-21

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